This invention relates to a filament cooler, particularly though not necessarily, a cooler for cooling an optical fibre freshly drawn from glass preform and requiring to be quickly cooled to a temperature suitable for the application thereto of a plastics protective coating. Such a cooler comprises a refrigerated jacket surrounding a portion of the fibre. The jacket is cooled by a flow of refrigerant through its walls, and this in turn cools the fibre through the agency of the thermal conductivity of a gaseous medium present between the fibre and in the inner wall of the refrigerated jacket.
In the case of cooling freshly drawn optical fibre, fast and efficient cooling is particularly desirable to enable the fibre to be capable of being drawn at high speed without having to have recourse to an excessively tall drawing tower or an inconveniently long cooler. Typically it is a requirement that the freshly drawn fibre be not allowed to come into contact with any surface before being provided with its protective coating. This means that the fibre can not be constrained to travel in a meander path through the cooler of greater length than that of the cooler, can not make multiple passes through the cooler, but must instead pass through it only once in a single straight line path. Having regard to the fact that the tension in the fibre over its unsupported length between the fibre drawing zone and the plastics coating applicator is determined by the required drawing conditions, it is evident that increasing this length in order to accommodate a longer cooler is going to have the effect of increasing the maximum amplitude of fibre vibration that is liable to occur over this length. This will mean that the clearance between the fibre and the surrounding inner wall of the jacket of the cooler will need to be greater in order to forestall the possibility of accidental contact, and this in turn means that cooling efficiency is reduced because the heat that has to be extracted from the fibre now has to be conveyed through the gas a greater distance from the fibre to the inner wall of the jacket.
Cooling efficiency is increased by the use of a lower temperature refrigerant, but generally there is a practical lower limit to the temperature of refrigerant that can be satisfactorily used, this being normally set by the need to operate at a temperature above the dew point in order to preclude problems associated with condensation on to surfaces of the cooler.
Cooling efficiency is also affected by the thermal conductivity of the gas present between the fibre and the inner well of the jacket of the cooler. Helium is especially effective in this respect, and has the additional advantage of being inert. On account of the cost of helium, a practical consideration in the design of an efficient cooler employing helium as coolant should preferably also take into account the avoidance of extravagant wastage of this gas.
A simple form of optical fibre cooler described in EP 0 174 699A comprises a jacketed pipe, held with its axis vertical, down through which the fibre is passed while helium gas is caused to flow upwardly by virtue of its natural buoyancy from an inlet point near the bottom end of the tube. An increase in cooling efficiency is obtained by reducing the bore of the pipe, thereby shortening the distance over which the heat has to be conducted from the fibre to the inner wall of the pipe. Under these circumstances the downward movement of the fibre, particularly if drawn at high speed, can produce a viscous drag overcoming the natural buoyancy of the helium, in which case the helium is introduced near the top of the tube, as for instance described in GB 2 226 310 A, rather than near the bottom.
The problem of obtaining increased cooling efficiency and reduced consumption of coolant are also addressed in U.S. Pat. No. 4,966,615 which discloses a water-jacketed pipe provided with a series of internal baffles which divide the interior into a set of communicating chambers. It is explained that the arrangement of baffles and chambers creates turbulence in the coolant gas flow where the flow enters a chamber after passing through one of the baffles, and that such turbulence improves cooling efficiency by breaking up the laminar flow of coolant gas surrounding the fibre. Cooling efficiency is also said to be improved by allowing additional cooling of the coolant gas in its passage through the baffles by allowing it to pass, not only through their central apertures that are threaded by the fibre, but also through additional apertures surrounding those central apertures.